Ultra Low NOx Direct Vent (DS) water heaters utilize a co-axial (pipe inside a pipe) vent pipe that takes air in from the outside (outer vent) and vents combustion gases directly back to the outside (inner vent). It is the perfect choice for situations with insufficient combustion air.
Ultra Low NOx High Input Direct Vent (DH) water heaters offer greater recovery capabilities, larger gallon capacities and side connections for space heating. These too utilize a co-axial (pipe inside a pipe) venting system. No electricity required.